This invention relates to an air/fuel ratio detecting device for use in exhaust gases of internal combustion engines operated with either a lean mixture or a rich mixture.
In recent internal combustion engines and particularly in automotive engines, it is prevailing to control the air/fuel mixing ratio precisely to a predetermined optimum ratio by performing feedback control. For example, when a so-called three-way catalyst is used in the exhaust system to achieve reduction of NOx and oxidation of CO and HC simultaneously, the air/fuel ratio is controlled precisely to a stoichiometric ratio because this catalyst exhibits best conversion efficiencies in an exhaust gas produced by combustion of a stoichiometric air-fuel mixture. In the current feedback control systems for this purpose it is usual to provide a feedback signal by sensing changes in the content of oxygen in the exhaust gas since there is a determined relationship between the actual air/fuel ratio in the engine and the oxygen content in the exhaust gas.
It is well known and has been put into industrial practice to produce an oxygen-sensitive device for use in exhaust gases of internal combustion engines by utilizing a semiconductive oxide of a transition metal, such as titania TiO.sub.2, since the resistance of such a metal oxide varies in dependence on the partial pressure of oxygen in an environmental gas. Usually titania is used in the form of a sintered body containing a small amount of catalytic material typified by platinum, and in principle an oxygen sensing or air/fuel ratio detecting device can be produced by attaching a pair of electrodes to the sintered titania body to measure the resistance of titania between the two electrodes. In exhaust gases of an internal combustion engine, the resistance of such a titania device remains nearly constantly at a low level while the engine is operated with a fuel-rich air-fuel mixture, but the resistance of this device shifts to a very high level when the engine is operated with a lean mixture. In other words, the resistance or output of the titania device in the exhaust gases exhibits a sharp and great change when the air/fuel ratio in the engine changes across a stoichiometric ratio from a higher value to a lower value or reversely. Accordingly this device is suitable for use in a feedback control system aiming at a stoichiometric air/fuel ratio in an internal combustion engine.
Meanwhile, so-called lean-burn engines in which the air/fuel mixing ratio is considerably higher than a stoichiometric ratio have been developed and put into practice with a view to attaining maximal thermal efficiency. Also, so-called rich-burn engines in which the air/fuel mixing ratio is considerably lower than a stoichiometric ratio have attracted attention because of the possibility of achieving a very high mechanical efficiency and have already been put into industrial practice where recirculation of exhaust gas is employed as a means for decreasing the emission of NOx. Accordingly there is a demand for an air/fuel ratio detecting device which is of use in exhaust gases of internal combustion engines operated with either a fuel-rich mixture or a lean mixture.